This invention relates generally to vapor-phase epitaxy for the formation of thin films of semiconductor compounds, and more particularly, to an improved method and means for effecting hydride vapor-phase epitaxy.
Epitaxy is the formation of a crystal on a crystalline substrate so as to replicate in the deposited material the crystalline structure of the substrate. Vapor-phase epitaxy is the formation of an epitaxial layer of a substance or substances by using a gaseous carrier to transport into a reaction chamber the substance or substances to be deposited epitaxially. A typical example of a structure suitable for epitaxial deposition is disclosed in Burd et al., U.S. Pat. No. 3,441,000, "Apparatus and Method for Production of Epitaxial Films." Burd et al. disclose a reactor for hydride vapor-phase epitaxy having three zones or chambers; a reaction chamber, a mixing chamber, and a deposition chamber. Burd et al. state that the gases are introduced into the various chambers at a relatively high flow rate through nozzles or orifices so that considerable mixing results. Gases which may be used are hydrogen, phosphine or arsine, and hydrogen chloride. The mixed gases react to produce indium chloride or gallium chloride which serves as a carrier for the gallium or indium that is deposited epitaxially as a phosphide or arsenide on a substrate in the deposition chamber. A problem that has limited the use of hydride vapor-phase epitaxy is that high component flow rates cause excessive growth rates. As a result, the crystal structure that is produced is often not of device quality. Attempts to throttle down the flow rate of reactants tend to interfere with epitaxial growth and to lead to attack by the excess of hydrogen chloride that results, leading to etching of the surface that is subject to epitaxial growth. This etching also interferes with the epitaxial growth.
Another example of vapor-phase epitaxy is disclosed in Jolly, U.S. Pat. No. 3,930,908, "Accurate Control During Vapor Phase Epitaxy." Jolly teaches control of the flow rate of a gaseous reactant by the use of a rotary valve which permits flow of the reactant to be substantially continuous, but which either directs the flow of the reactant into the carrier stream or else directs that flow to be vented. Jolly suggests the desirability of adding an additional impurity in the form of a gaseous reactant, such as a conductivity modifier, into the reaction chamber in order to form a layer of the semiconductor material in which the conductivity modifier is present. Jolly further suggests that it is necessary to minimize the time it takes to introduce particular gaseous components into the reaction chamber in order to form the desired layers as quickly as possible. In his teachings, Jolly does not mention as of concern the difficulty of achieving substantially perfect replication of the lattice of the substrate. The quality of semiconductor devices formed by vapor-phase epitaxy is determined by the essentially complete absence of grain boundaries, so that the subject matter of epitaxial growth is a single crystal. Another figure of merit is represented by the substantial absence of dislocations within the crystalline structure. Such dislocations represent crystal defects which in general are detectable by measuring the mobilities of electrons and holes in the semiconductor material. It has been found that the best replication of crystalline structure is carried out at deposition rates of the order of 100 Angstroms per second or less, with good results being achieved at deposition rates of the order of 25 Angstroms per second or less. It is difficult to achieve deposition rates this low with steady-state flow of the type taught by Jolly.
Fraas, U.S. Pat. No. 4,146,774, "Planar Reactive Evaporation Apparatus for the Deposition of Compound Semiconducting Films," discloses epitaxial formation of compound semiconductor films, and particularly the formation of indium phosphide and gallium phosphide. Fraas teaches a structure for evaporation of the metallic component of a compound semiconducting material into a carrier stream that contains the nonmetallic element in compound form. Fraas requires residual gas pressures of less than 10.sup.-9 Torr, which are obtained by operation in a high-vacuum chamber and by employing a shroud that is cooled by liquid nitrogen. The necessity for pumping components into a high vacuum represents a disadvantage of the apparatus taught by Fraas.
Duchemin et al., U.S. Pat. No. 4,220,488, "Gas-Phase Process for the Production of an Epitaxial Layer of Indium Phosphide," teach a two-step reaction involving pyrolysis of phosphine to produce phosphorous which is reacted with triethyl indium to produce indium phosphide which is then deposited epitaxially on a substrate. This reaction is carried at a pressure which is low relative to atmospheric pressure and specifically is in the range of 70 to 80 Torrs. This process is referred to as organometallic vapor-phase epitaxy, and is widely used to produce epitaxial growth.
Blakeslee, U.S. Pat. No. 3,721,583, "Vapor-Phase Epitaxial Deposition for Forming Superlattice Structure," teaches a method of using a vapor-phase epitaxial deposition system for forming the physically abrupt layers that are necessary in superlattices. Blakeslee accomplishes this by injecting a particular component into the flowing stream when that component is to be deposited epitaxially and withholding that component from the flowing stream when its deposition is not desired. While Blakeslee refers to this as injecting pulses of the n component in a carrier gas separated by pulses of carrier gas into the n-1 component stream, it appears that his pulses are of sufficient length to supply one pulse per layer of the component in the superlattices. Accordingly, this method is different from the method disclosed and claimed below by the applicant.
In short, vapor-phase epitaxial deposition of semiconductor ingredients is well known. The objective of such processes is to achieve materials of desired crystalline structure and composition in the form of single crystals having no grain boundaries, essentially no or a very small number of crystal dislocations, and a controlled minimum number of unintentional dopants. The presence of unintended dopants is controlled by controlling the purity of carrier gases and reacting elements to limit the presence of undesired components. However, it has been observed that, to obtain device-quality semiconductors through epitaxial growth, the growth rate must be limited to minimize the development of an intolerable number of dislocations. For a steady-state supply of reacting components, it is often difficult to throttle the supply of components to a low enough level to provide a satisfactory epitaxial growth rate for high-quality thin-film applications.